Ribbon foil depressor

ABSTRACT

A ribbon-foil depressor is incorporated into a towed seismic array to provide downward lift to array sections or components. The ribbon-foil depressors may be deployed on the port and starboard spur lines or on the outboard separation ropes. The ribbon-foil depressors may be used to submerge and operate seismic equipment at depths as low as 60 m or more and are capable of maintaining towed seismic streamer cables at these depths and still remain stable through various speed changes and turns.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority pursuant to 35 U.S.C. §119(e) of U.S. provisional application No. 62/295,561 filed 16 Feb. 2016entitled “Ribbon-foil depressor,” which is hereby incorporated herein byreference in its entirety.

TECHNICAL FIELD

The technology described herein relates to designs for fairings andfoils attached to tow lines and cables for submerging sensorarrangements used in marine seismic exploration.

BACKGROUND

In towed marine seismic exploration, a hydrophone array is typicallytowed behind a marine vessel near the sea surface. The hydrophones aremounted to multiple sensor cables, commonly referred to as streamers.The streamers serve as platforms or carriers for the hydrophones, whichare distributed along the length of each streamer in the array.

A set of seismic sources, also towed near the sea surface, are operatedto periodically emit acoustic energy. The acoustic energy of interestpropagates downward through the seawater (or other water column),penetrates the ocean floor, reflects from the subsea strata and otherunderlying structures, and returns upward through the water column tothe hydrophone array.

The reflected seismic energy (or acoustic wave energy) arrives atreceiver points in the towed hydrophone array. The array includes manysuch receiver points, distributed along each of the streamer cables,with sensors configured to generate data records characterizing theupward-traveling acoustic wavelets (or seismic waves) received from thesubsurface structures beneath the seabed, at each of the receiverpoints. The hydrophone data recordings are later processed to generateseismic images of the underlying structure

In the field of subsea seismic exploration, there has recently been ademand for seismic equipment operators to conduct their surveys with theseismic equipment submerged below the depths at which most seismicsurveys have been conducted in the past. These new, deeper operatingtargets can now lie well below the depth of the surface-referencedequipment (i.e., the vessel and the paravanes) that is used to tow andlaterally spread the seismic sensors.

Typical marine depressors for maintaining equipment at a substantiallyconstant submerged depth tend to be fairly small with very poor aspectratios thus resulting in low lift. Aspect ratio is defined as the spanof the depressor divided by its chord line length. Wings with highaspect ratios generate high downward lift forces for minimal drag (suchthat lift-to-drag ratios as high as 10:1 or more are possible), whereaswings with aspect ratios as low as 1 or 2 (i.e., where span and chordare roughly the same scale) will typically have lift-to-drag ratios aslow as 2:1, or even lower. Conventional depressors (see, e.g., HydroForce Technologies AS “HFT Catfish 100” (<http://www.hft.no/catfish/>),or YSI Incorporated “V-Fin”(<https://www.ysi.com/File%20Library/Documents/Specification%20Sheets/E72-Standard-V-fins.pdf>)often also provide payload bays which can be used to hold additionalballast to supplement the downforce generated by the depressor.

The problem with using deadweight to generate downforce is that it doesnot scale with tow speed—it provides a constant downforce regardless ofhow fast the depressor is moving through the water. This is oftendisadvantageous for those applications where a range of operationalspeeds is expected, with the requirement that the towed equipmentmaintain a stable depth over that speed range. Consequently, there is noeasy, economical, or ideal way to submerge and operate seismicequipment, such as towed streamer cables, at the desired lower depths.

The information included in this Background section of thespecification, including any references cited herein and any descriptionor discussion thereof, is included for technical reference purposes onlyand is not to be regarded subject matter by which the scope of theinvention as defined in the claims is to be bound.

SUMMARY

To achieve the objective of submerging and operating seismic equipmentat the desired depths (i.e., below the midpoint depth of the paravanes,for example), a specialized hydrofoil, or ribbon-foil depressor may beincorporated into the towed seismic array. The ribbon-foil depressorsare preferably deployed on the port and starboard spur lines.Alternatively, the ribbon-foil depressors may be mounted on the outboardseparation ropes. These ribbon-foil depressors may be used to submergeand operate seismic equipment at depths as low as 60 m or more and arecapable of maintaining towed seismic streamer cables at these depths andstill remain stable through various speed changes and turns.

One potential application is the depression of outboard streamer headsin seismic arrays, relative to the depth of the paravane crucifixes towhich the spur lines are attached. In this application, the ribbon-foildepressors generate sufficient downforce to cause the spur line catenaryto curve downward to the desired streamer depth, over the span of 10 to100 meters, depending on the length of the spur lines used.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. A moreextensive presentation of features, details, utilities, and advantagesof the present invention as defined in the claims is provided in thefollowing written description of various embodiments of the inventionand illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a schematic illustration of a first exampleof a towed, three-dimensional, high-resolution seismic array.

FIG. 2 is a rear elevation view of a schematic illustration of the towedseismic array of FIG. 1.

FIG. 3 is an enlarged, partial rear elevation view of a schematicillustration of a port side of the towed seismic array of FIG. 1 with asystem of ribbon-foil depressors provided on the spur line.

FIG. 4 is a top plan view of a schematic illustration of a secondexample of a towed seismic array.

FIG. 5 is a rear elevation view of a schematic illustration of the towedseismic array of FIG. 4.

FIG. 6 is an enlarged, partial rear elevation view of a schematicillustration of a port side of the towed seismic array of FIG. 4 with asystem of ribbon-foil depressors provided on the spur line.

FIG. 7 is a rear elevation view of a schematic illustration of the towedseismic array of FIG. 4 with a system of ribbon-foil depressors providedon the port and starboard separation lines.

FIG. 8A is an isometric view of a single ribbon-foil depressor section.

FIG. 8B is a left side elevation view of the ribbon foil depressorsection of FIG. 8A.

FIG. 9 is a model depicting deflection of a ribbon-foil depressor on acable as compared to standard symmetrical fairings.

FIG. 10 is a plot of angle of attack of an exemplary ribbon-foildepressor section as compared to the pivot location of the ribbon-foildepressor section.

FIG. 11 is a plot of the lift coefficient of an exemplary ribbon-foildepressor section as compared to the pivot location of the ribbon-foildepressor section.

FIG. 12 is a plot of the moment coefficient of an exemplary ribbon-foildepressor section as compared to the pivot location of the ribbon-foildepressor section.

FIG. 13A is a rear elevation view of a schematic illustration of aribbon-foil depressor array on a bar configured for attachment by abridle to a cable.

FIG. 13B is a port side elevation view of a schematic illustration ofthe ribbon-foil depressor array of FIG. 13B.

FIG. 14 is port side elevation view of a schematic illustration of aseries of ribbon-foil depressor arrays mounted in fixed positions alonga cable.

FIG. 15 is port side elevation view of a schematic illustration of aribbon-foil depressor array in a variable position mount along a cable.

FIG. 16 is an enlarged, partial rear elevation view of a schematicillustration of a port side of a towed seismic array with a system ofribbon-foil depressors provided on the spur line and connected to theport paravane by a bridle.

DETAILED DESCRIPTION

Seismic arrays with sources and streamers are used to study rock strataand other structures below the surface of the ocean or other bodies ofwater. One or more marine vessels are typically used to tow the sourceand/or receiver arrays, in order to obtain relevant geological datacovering a desired surface area of the ocean floor. For example, asingle surface vessel may simultaneously tow both a source array and anarray of seismic streamers, or different vessels can be used to towseparate source and receiver arrays. Alternatively, a towed source arraycan be used in conjunction with stationary receivers, for example, anarray of ocean-bottom nodes, or with ocean-bottom cables deployed on theseabed.

During operation, acoustic shock waves generated by the source arraypropagate through the water to penetrate the ocean floor and arereflected from subsurface structures. The reflected acoustic waves arerecorded as signals or seismic responses by the receivers, e.g.,hydrophones and/or geophones towed behind a vessel or deployed on theocean floor.

Lateral forces are applied to maintain position and spacing of theseismic sources and other array elements as they are towed behind thevessel. The spacing depends on the number of sources and/or streamercables that are deployed, and on the spacing between adjacent sourceand/or receiver components. Typically, a number of source sub-arrays orstrings are deployed behind the vessel using a tow rope configuration tospread the sources over lateral distances of approximately ten to onehundred meters or more. Streamer cables are typically deployed over muchlarger lateral distances, for example, from one hundred meters to akilometer or more, and may extend for several kilometers behind the towvessel.

Lateral spacing can be achieved by deploying a paravane or diverterapparatus on a dedicated tow rope arrangement using a spreader or seriesof individual tether lines to provide the desired spacing betweenadjacent cables. Positioning devices can also be provided along eachstreamer cable, in order to maintain depth and/or lateral offset alongthe cable length.

One embodiment of a towed three-dimensional, high-resolution, marineseismic array 100 is depicted in FIGS. 1 and 2. The array 100 is towedby a marine vessel 102. A number of cables, ropes, or other lines may beattached to the marine vessel 102. For example, an umbilical cable 104with acoustic signal source generators (e.g., air guns) may traildirectly behind the marine vessel 102. A pair of tow ropes 106 or cablesmay splay out to port and starboard from the rear of the marine vessel102. A cross-cable 108 may extend between and connect to the tow ropes106 adjacent the aft ends of the tow ropes 106. A number of streamercables 110 may be connected to the cross-cable 108 at a number oflocations along the length of the cross-cable 108 between the tow ropes106. In some embodiments, the streamer cables 110 may be evenly spacedapart from adjacent streamer cables 110 along the length of thecross-cable 108. In a typical embodiment, there may be up to 18 streamercables 110 and they may be spaced anywhere between 10 m and 100 m ormore apart. Respective tail buoys 111 may be affixed to the ends of eachof the streamer cables 110 to aid in maintaining a constant depth of thestreamer cables 110 below the surface.

The cross-cable 108 may extend beyond the port-most and starboard-moststreamer cables 110 to attach to the tow ropes 106. These lateralsections of the cross-cable 108 may be referred to as spur lines 114. Insome embodiments, the spur lines 114 may be separate ropes or cablesthat connect to and extend between the lateral ends of the cross-cable108 and the tow ropes 106.

Paravanes 112 may further be attached to the tow ropes 106 at oradjacent to the connection between the tow ropes 106 and the spur lines114 on each of the port and starboard sides. The paravanes 112 arewinged hydrofoils that move outward in the water in an oblique directionto the direction of travel of the marine vessel 102, thus providinglateral spread to the cross-cable 106 and the streamer cables 110attached thereto.

A signal cable 116 may extend from the marine vessel 102 on one side ofthe array 100 to connect to the cross-cable 108 and return signalsreceived by the sensors 113 on the streamer cables 110. On an oppositeside of the array 100, a recovery rope 118 may extend from the marinevessel 102 and connect with the cross-cable 108 adjacent to the laststreamer 110. Surface floats 117 may be attached to the cross-cable 108at or adjacent to the lateral ends thereof via a cable with a lengthcorresponding to a desired depth of the streamer cables 110. The surfacefloats 117 act to ensure that the cross-cable 108, and thus the streamercables 110, do not submerge too deeply when the array 100 is towed.

Unfortunately, the port and starboard ends of the cross-cable 108, andthus the streamer cables 110 attached thereto, may not achieve a desireddepth beneath the surface due to the pull of the paravanes 112 on thespur lines 114. The paravanes 112 remain at the surface of the water andthus pull the lateral ends of the cross-cable upward as well aslaterally outward.

A number of positioning devices or depressors 120 designed to providedownward lift to counteract the effect of the paravanes 112 on thecross-cable 108 may be attached to the cross-cable 108, the spur line114, or both. These depressors 120 may be shaped as foils and may bepivotably attached to the cross-cable 108 or the spur line 114 to moveindependently of each other, or in concert in some embodiments. As thedepressors 120 are independent of each other, they may evoke anappearance resemblant of a ribbon fairing for a cable. The depressors120 may thus be referred to herein as “ribbon-foil depressors.” As shownin FIG. 3, the depressors 120 may fill the entire length of the spurline 114. Alternatively, the depressors 120 may only fill a portion ofthe spur line 114 and may be situated either laterally outward closer tothe paravanes 112 or more inward closer to the streamer cables 110. Asnoted above, the depressors 120 may also be positioned on thecross-cable 108, inside the port-most and starboard-most streamer cables110. The location of the depressors 120 may be selected based upon anumber of factors including the amount of downward lift generated by thedepressors 120, the separation distance of the streamer cables 110, themass of the sensors 113, streamer cables 110, and cross-cable 108, andthe lift force generated by the paravanes 112 among other factors.

Another example of a towed seismic array 200 is presented in FIGS. 4 and5. FIG. 4 depicts the array 200 from a top plan view and indicates thedistance of the tow cables 206 and tether or lead-in cables 209 from thestern of the marine vessel (not shown) on the vertical axis and thus thespread distance between the streamer cables (not shown) connected to theaft ends of the lead-in cables 209 on the horizontal axis. FIG. 5depicts the array 200 from a rear elevation view and indicates the depthof position of the aft ends of the lead-in cables 209 and,correspondingly, the depth of the forward ends of the streamer cablesbelow the head-end floats 211 on the vertical axis and the spreaddistance between the aft ends of the lead-in cable and forward ends ofthe streamer cables (not shown) on the horizontal axis. As in the priorembodiment, a number of umbilical cables 204 with acoustic signal sourcegenerators may trail directly behind the marine vessel.

In this embodiment, the forward ends of the streamer cables 210 attachat a single point rather than at a number of points spread along across-cable. The aft ends of each adjacent pair of the lead-in cables209 are connected together by separation ropes 208, which may be between25 m and 200 m or more in length and thus may extend similarly as faraft of the marine vessel such that the sensors are generally positionedalong a horizontal line. Surface floats 217 may be attached to theseparation ropes 208 at or adjacent to the port-most and starboard-moststreamer cables 210 via a cable with a length corresponding to a desireddepth of the streamer cables 210. The surface floats 217 act to ensurethat the streamer cables 110, do not submerge too deeply when the array200 is towed.

Spur lines 214 may extend from the aft ends of each of the port-most andstarboard-most lead-in cables 209, respectively, and attach to a pair oftow ropes 206 extending along the lateral sides of the array 200. Thespur lines 214 may be up to 75 m in length or more. The paravanes 212may further attach to the connection point between the spur lines 214and the tow ropes 206 to spread the aft ends of the lead-in cables 209apart.

As depicted in FIG. 5, the paravanes 212 effect an upward lift on thespur lines 214 and thus raise the aft ends of the port and starboardlead-in cables 209, and thus the streamer cables connected thereto,above the desired depth at which the middle streamer cables 210 extend.Therefore, according to an alternate implementation, a series ofribbon-foil depressors 220 may be attached to the spur lines 214 asshown in FIG. 6. Such ribbon-foil depressors 220 may extend the entirelength of the spur lines 214 or only a portion thereof. If theribbon-foil depressors 220 extend only a portion of the length of thespur lines 214, they may be located adjacent to the bridle connectionfor the paravane 212, adjacent to the port-most and starboard-mostlead-in cables 209, or in between the respective paravane 212 andlead-in cable 209. Alternatively, the depressors 120 may be positionedon the separation rope 208 inside the port-most and starboard-moststreamer cables 210 or on both the spur lines 214 and the separationropes 208. The location of the depressors 220 may be selected based upona number of factors including the amount of downward lift generated bythe depressors 220, the separation distance between the lead-in cables209, the mass of the lead-in cables 209, and the lift force generated bythe paravanes 212 among other factors.

FIG. 7 schematically depicts the effect of the ribbon-foil depressors220 on the array 200 when attached to the separation ropes 208 insidethe port-most and starboard-most streamer cables 210. The aft ends ofthe port-most and starboard-most lead-in cables 209 are submerged to thedesired depth (in this example, 14 m) to coincide with the depth of themiddle lead-in cables 209. The ribbon-foil depressors 220 therebyprovide downward lift on the separation ropes 208 to counter the upwardlift effects of the paravanes 212 and maintain submergence of thelead-in cables 209, and thus the streamer cables, at an appropriatedepth.

An exemplary form of a single depressor section 322 of a ribbon-foildepressor is depicted in FIGS. 8A and 8B. The depressor section 322 isscalable to suit a wide range of lift requirements, while also offeringvery high aspect ratios and avoiding any requirement for supplementaryballast. The depressor section 322 has a body 330 with a foil shapehaving a leading edge 332 and a trailing edge 334. The line connectingthe leading edge 332 and the trailing edge 334 passing through themid-thickness of the body 330 is referred to as the “chord line” of thefoil shape. When viewed from a top plan perspective, the depressorsection 322 may appear rectangular in shape. A first surface 336 extendsbetween the leading edge 332 and the trailing edge 334 and may becambered. A second surface 338 of the body 330 extends between theleading edge 332 and the trailing edge 334 and may be relatively flatwith respect to the first surface 336.

A tail flap 340 may form a portion of the trailing edge 334. The tailflap 340 may extend above and aft of the first surface 336 and form anobtuse angle with respect to the first surface 336. A tail flap 340 canbe created by “bending” the body 330 at the chord line at some discretedistance forward of the trailing edge 334. In some embodiments, the tailflap 340 extends across between 5% and 25% of the length of the chordline of the depressor section 322. In some embodiments, the bend angleof the tail flap 340 can be a departure of 5° up to 20° or 30° away fromthe axis of the original chord line. This results in the obtuse anglebetween the first surface 336 and the tail flap 340 being between 150degrees and 175 degrees. The combination of the camber of the firstsurface 336 and the length and angle of the tail flap 340 may beconfigured to provide negative lift as further described below.

“Bending” the aft portion of the depressor section 322 in this fashionrepresents only one possible method for creating a tail flap 340. Otherways may include truncating the original depressor section 322 byremoving some portion of the trailing edge 334, and replacing it withanother trailing section having a chord line that departs from the chordline of the leading edge 332 of the body 330. Alternatively, theoriginal foil shape of the depressor section 322 could be left entirelyintact while a separate, additional trailing edge section is appended tothe original foil shape, either directly behind the trailing edge 334 ofthe depressor section 322, or at some distance downstream of thetrailing edge 334 of the depressor section 322.

The body 330 has two lateral sides 342, 344 that extend between thelateral edges of the first surface 336 the second surface 338 andbetween the leading edge 332 and the trailing edge 334. The body 330 maybe made from solid cast polyurethane for near-neutral buoyancy and highabrasion resistance and durability. However, the body 330 may still beslightly negatively buoyant, such that the body 330 will influence theequilibrium angle of attack, especially at low tow speeds. Thus, thedownforce achieved by the depressor section 322 may be influenced byselecting the composition of the body 330.

A first tubular conduit 346 may be defined within the body 330 andextend laterally through the body 330 adjacent to the leading edge 332and open to each of the first and second lateral sides 342, 344. Thefirst tubular conduit 346 is sized to receive ropes or cables (such asseparation ropes and/or spur lines) of a seismic array therethrough. Asecond tubular conduit 348 may be defined within the body 330 and extendlaterally therein adjacent to, aft of, and parallel to the first tubularconduit 346 and open to each of the first and second lateral sides 342,344. The second tubular conduit 348 is similarly sized to receive ropesor cables of the seismic array therethrough. A third tubular conduit 350may be defined within the body 330 and extend laterally therein adjacentto, aft of, and parallel to the second tubular conduit 348 and open toeach of the first and second lateral sides 342, 344. The third tubularconduit 350 may be similarly sized to receive ropes or cables of theseismic array therethrough. Each of the first, second, and third tubularconduits 346, 348, 350 may be positioned within the forward 50 percentof the chord length of the depressor 322.

A fourth tubular conduit 352 may be defined within the body 330 andextend laterally therein aft of and parallel to the third tubularconduit 350 and open to each of the first and second lateral sides 342,344. The fourth tubular conduit 352 may be positioned within the aft 50percent of the of the cord length of the depressor 322 and forward ofthe beginning of the tail flap 340. The fourth tubular conduit 352 maybe similarly sized to receive a rope or cable therethrough. The fourthtubular conduit 352 may alternatively be filled with syntactic foam orother more buoyant material to help counteract the negative buoyancy ofthe polyurethane material forming the body 330.

According to one embodiment, the of FIGS. 3 and 6 may be configured as astring of segmented, cambered, foil depressor sections 322, eachpreferably with a tail flap 340. The ribbon-foil depressor 120, 220 maybe a passive, non-steerable depressor for the depression of outboardstreamer heads in seismic arrays, relative to the depth of thecrucifixes of the paravane 112, 212 to which the spur lines 114, 214 areattached. In this implementation, the ribbon-foil depressors 120, 220generate sufficient downforce to cause the catenary of the spur lines114, 214 to curve downwards to the desired streamer depth, over the spanof 10 to 100 meters, depending on the length of the spur lines 114, 214in operation. This downward curve is modeled in FIG. 9 in which aribbon-foil depressor 420 is mounted on a rope or cable in a flume tank.The downward depression of the rope on which the ribbon-foil depressor420 is mounted is clear in comparison to the symmetric fairings 450mounted on rods extending across the test fixture in the flume tank. Inthis embodiment, the depressor sections 322 may be threaded onto eithera rope or rigid rod through one of the first, second, or third tubularconduits 346, 348, 350 in the forward half of the chord length of thebody 330.

The ribbon-foil depressor 120, 220 is scalable to suit a wide range ofnegative lift requirements, while also offering very high aspect ratiosand avoiding any requirement for supplementary ballast. The depressorsections 322 of may be free to rotate in a flow field. The angle ofattack at which the ribbon-foil depressor 120, 220 will achieveequilibrium will be a function of the center of rotation about which thedepressor sections 322 pivot, and the moment coefficient of theparticular airfoil shape of the depressor section 322 being used. Thecenter of rotation for the depressor sections is the one of the first,second, or third tubular conduits 346, 348, 350 toward the leading edge332 of the body 330 through which the rope or bar passes.

Most cambered airfoils shapes, such as a NACA2318, for example, willhave a moment coefficient that is negative, meaning that the airfoilwill find an equilibrium angle of attack that is negative when allowedto freely rotate about a pivot point. In order to create a positivemoment coefficient, i.e., in order to generate a positive angle ofattack and achieve high maximum downward lift with greater lift-to-dragefficiency, the tail flap 340 may be added to the airfoil camber of thefirst surface 336 of the body 330. In some embodiments, the depressorsections 322 may conform to the NACA2318 standard shape and have amaximum camber of 2% located at 30% of the chord line (or 0.3. of thechord line length from the leading edge 332) with a maximum thickness of18% of the chord line length.

The multiple tubular conduits 346, 348, 350 are provided for flexibilityin selecting a pivot point. Inserting a rope or rod into any of thethree forward tubular conduits 346, 348, 350 means that the depressorsections 120, 220 will freely pivot on that center of rotation and reachits own equilibrium angle of attack in the flow field. The equilibriumangle of attack achieved by the depressor sections 120, 220 configuredpassively in forming the ribbon-foil depressor 120, 220 may bedetermined through a selection of the following parameters:

-   -   Location of the pivot, i.e., the tubular conduit 346, 348, 350        through which the rope passes relative to the leading edge 332        of the body 330;    -   The particular foil shape of the depressor sections 322 (or a        symmetric foil shape in combination with a suitable tail flap        340),    -   The angle of attack of the tail flap 340 (if any); and    -   The percentage of chord line of the tail flap 340 (if any).

As shown by the plot graph of FIG. 10, the length and angle of the tailflap 340 and the pivot location together directly influence theresultant equilibrium angle of attack of the depressor sections 322. Thegreater the length and angle of the tail flap 340, the greater the angleof attack. When the pivot point is close to the leading edge 332, theeffect on the angle of attack is minimal, but as the pivot point movesaft of the leading edge 332, the effect on the angle of attack becomespronounced.

Similarly, as shown in FIG. 11, the length and angle of the tail flap340 and the pivot location together directly influence the resultantlift coefficient of the depressor sections 322. The greater the lengthand angle of the tail flap 340, the greater the lift coefficient. Whenthe pivot point is close to the leading edge 332, the effect on the liftcoefficient is minimal, but as the pivot point moves aft of the leadingedge 332, the effect on the lift coefficient becomes pronounced.

Additionally, as shown in FIG. 12, the length and angle of the tail flap340 and the pivot location together directly influence the resultantmoment coefficient of the depressor sections 322. The greater the lengthand angle of the tail flap 340, the greater the moment coefficient. Whenthe pivot point is close to the leading edge 332, the effect on themoment coefficient is minimal, but as the pivot point moves aft of theleading edge 332, the effect on the moment coefficient becomespronounced and the moment coefficient is driven toward 0. If the pivotlocation is well forward and no tail flap 340 is included, then acambered foil shape of the depressor section 322, as shown in FIGS. 8Aand 8B, will actually find a negative angle of attack at equilibrium.Moving the pivot point slightly aft and introducing a small tail flap340 will cause the moment coefficient to change signs, such that thedepressor section 322 will find its equilibrium at a positive angle ofattack.

Consequently, the magnitude of downforce generated by the ribbon-foildepressor 120, 220 formed by depressor sections 322 can be controlled byvarious factors including the following:

-   -   Adjusting the overall span of the ribbon-foil depressor 120, 220        (i.e. the number of depressor sections 322 threaded onto the        rope or rod);    -   Varying the length of the chord of the depressor sections 322        (i.e. customize the size of the depressor sections 322 at time        of manufacture to suit the required end application);    -   Selection of the pivot location with respect to the leading edge        on the depressor sections 322 for the rope or rod (i.e., the        further aft the pivot point is of the leading edge 332, the        higher the resulting angle of attack of the depressor section        322);    -   Varying the angle of attack of the tail flap 340;    -   Varying the size of the tail flap 340 (i.e., selection of the        percentage of chord of the tail flap 340), and    -   Choice of camber for the foil profile of the depressor sections        322 (lesser or greater cambered foil depressor sections 322        generate lower or higher lift coefficients).

In an alternate implementation, a second rope or cable may be threadedthrough the fourth tubular conduit 352 in the depressor sections 322 ofthe ribbon-foil depressor 120, 220. The second rope or cable allows foradjustment of the lift by controlling the catenary (billow) of theribbon-foil depressor 120, 220. In this embodiment, the pair of ropesmay be adjusted in length to effect controllable adjustments in lift.The equilibrium angle of attack achieved by this embodiment is afunction of the relative lengths of the dual ropes. For example, if theaft rope passing through the fourth tubular conduit 352 in the aft halfof the depressor sections 322 is shortened with respect to the ropepassing through the selected pivot point in one of the first, second, orthird tubular conduits 346, 348, 350, the trailing edges 334 of thedepressor sections 322 will be pushed closer together laterally ascompared to spacing between the depressor sections 322 at the leadingedges 332. This causes the ribbon-foil depressor 120, 220 to billow andchange the angle of attack along the length of the ribbon-foil depressor120, 220.

Another potential application of the ribbon-foil depressor 420 is todepress source umbilical cables 404 by mounting the ribbon-foildepressor 420 on a swing seat 450 as depicted in FIGS. 13A, 13B, and 14.In this case, depressor sections 422 may be installed onto a bar orrigid rod 424, supported at either end by a Y-bridle 426, therebyforming the swing seat 450. In some embodiments, a third cable may beincluded in the Y-bridle 426 to attach to a midpoint of the rigid rod424. As shown in FIG. 13A, two depressor sections 422 are threaded ontothe rigid rod 424, which is suspended from a cable connector 428 bythree cables forming the Y-bridle 426. The cables of the Y-bridle 426may connect to a shackle 430 pivotably attached to the cable connector428, as shown in FIG. 14. The cable connector 428 may be formed as aclam shell clamp 432 that clamps around the umbilical cable 404. An eye434 may be mounted (e.g., by a weld joint) to a bottom of the clamp 432through which the bolt 436 of the shackle 430 may be fitted. In someembodiments, a bend restrictor (e.g., a Cumberland grip) (not shown) maybe placed around the umbilical cable 404 at a desired attachmentlocation and the clamp 432 may be seated around the bend restrictor inorder to protect the umbilical cable 404 and provide a compressionsurface for the clamp 432 to affix against. A number of ribbon-foildepressors 420 mounted on swing seats 450 may thus be deployed atdiscrete, fixed locations along the length of the umbilical cable 404.

In the case of the spur line application, shown in FIGS. 3 and 6, theaspect ratio of the depressor sections of the ribbon-foil depressor 120,220 can be as high as 100-to-1 or even up to 1000-to-1. In the case ofthe ribbon-foil depressors 420 mounted on swing seats 450 as shown inFIGS. 13A, 13B, and 14, the aspect ratio may be as low as 5-to-1.However, this is still an improvement over typical depressors withaspect ratios as low as 1-to-1. As mentioned above, the aspect ratio ofthe foil shape of the depressor sections 422 is a critical factor indetermining the lift-to-drag efficiency of the depressor sections 422and of the ribbon-foil depressor 420 as a whole.

In an alternate embodiment shown in FIG. 15, ribbon-foil depressors 420mounted on swing seats 450 may be adjustably mounted to the umbilicalcable 404. In this embodiment, a diameter of the clam shell clamp 432 ofthe cable connector 428 may be larger than the diameter of the umbilicalcable 404, thereby allowing the cable connector 428 to slide along theumbilical cable 404. In other embodiments, the cable connector 428 mayinclude a sliding mechanism, e.g., one or more pulleys or bearingsconfigured to roll along or against the umbilical cable 404 The cableconnector 428 may be attached at a top connector ring 438 or other mountto a thin tagline 440 extending from a utility winch (not shown) mountedon an aft deck of the marine vessel. The position and depth of theribbon-foil depressor 420 along the umbilical cable 404 can thus becontrolled by adjusting the length of the tagline 440 to change theposition of the ribbon-foil depressor 420 along the umbilical cable 404,as shown in FIG. 15.

In addition to the depth control discussed above, a ribbon-foildepressor 520 deployed on the spur line 514 connected to the streamercable 510 below the surface float 517 may also provide “lift assist” tothe paravanes 512 attached by a bridle 513 to the intersection of thetow lines 504 and spur lines 514 as shown in FIG. 16. That is, since theribbon-foil depressor 520 induces a downward catenary to the spur line514, as shown in FIG. 16, a first component 562 of the lift force 560acts downward as discussed above, but a second component 564 of the liftforce 560 also acts horizontally (i.e. outboard). This horizontal “liftassist” of the second component 564 provided by the ribbon-foildepressor 520 means that the existing standard paravanes 512 will now beable to spread the seismic array 500 wider than previously possible.Alternatively, the configuration including the ribbon-foil depressor 520on the spur line 514 may achieve the same spread but at a shorter offsetbehind the marine vessel towing the array 500. In anotherimplementation, the same spread and offset may be achieved, but a moreefficient setting for the bridle 513 attaching the paravanes 512 may beused and hence reduce fuel consumption of the marine vessel.

In addition to the use of a series of depressor sections on spur linesto achieve depression forces to submerge streamer heads down to desireddepths for seismic arrays, the ribbon-foil depressor may provide anumber of other features and advantages.

The ribbon-foil depressor can readily be installed on existing in-waterequipment, such as, for example, by threading the individual depressorsections onto existing spur lines between paravanes and outboardstreamer cable heads. Ribbon-foil depressors may also be installed onnumerous other existing ropes or rods, such as, for example, on thestandard spreader bars of the dual gun clusters used for seismicsources.

The ribbon-foil depressor can be deployed over the side of the marinevessel, or down the gun chute, and will then self-orient and generatelift without operator intervention. Handling, deployment, and recoveryoperations are essentially hands-free with no special davits ordedicated winches or cranes required. It is also compact and can beeasily and efficiently stowed on the vessel when onboard.

The ribbon-foil depressor offers high aspect ratios and highlift-to-drag efficiency. The ribbon-foil depressor offers a high degreeof flexibility in terms of the number of choices available, includingpivot location, camber, chord length, and tail fin size and angle, toselectively adjust the downforce to suit operational requirements andspecifications. Lift is also adjustable by using a dual rope embodimentor providing a high buoyancy material in the aft tubular conduit.

Ribbon-foil depressors applied to umbilicals or other similar typecables can also be scaled by how many are deployed, for example, bydaisy-chaining depressor sections at intervals along the cable

All directional references (e.g., proximal, distal, upper, lower,upward, downward, left, right, lateral, longitudinal, front, back, top,bottom, above, below, vertical, horizontal, radial, axial, clockwise,and counterclockwise) are only used for identification purposes to aidthe reader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Connection references (e.g., attached, coupled, connected,and joined) are to be construed broadly and may include intermediatemembers between a collection of elements and relative movement betweenelements unless otherwise indicated. As such, connection references donot necessarily infer that two elements are directly connected and infixed relation to each other. The exemplary drawings are for purposes ofillustration only and the dimensions, positions, order and relativesizes reflected in the drawings attached hereto may vary.

The above specification, examples and data provide a completedescription of the structure and use of exemplary embodiments of theinvention as defined in the claims. Although various embodiments of theclaimed invention have been described above with a certain degree ofparticularity, or with reference to one or more individual embodiments,those skilled in the art could make numerous alterations to thedisclosed embodiments without departing from the spirit or scope of theclaimed invention. Other embodiments are therefore contemplated. It isintended that all matter contained in the above description and shown inthe accompanying drawings shall be interpreted as illustrative only ofparticular embodiments and not limiting. Changes in detail or structuremay be made without departing from the basic elements of the inventionas defined in the following claims.

What is claimed is:
 1. A depressor foil for submerging a towed cablecomprising a body with a foil shape having a leading edge, a trailingedge, which define a chord length therebetween and further defining acambered first surface extending between the leading edge and thetrailing edge configured to provide negative lift; a relatively flatsecond surface with respect to the first surface extending between theleading edge and the trailing edge; a first lateral side that extendsbetween a first lateral edge of the first surface, a first lateral edgeof the second surface, the leading edge, and the trailing edge; and asecond lateral side that extends between a second lateral edge of thefirst surface, a second lateral edge of the second surface, the leadingedge, and the trailing edge; a first tubular conduit configured toreceive a cable therethrough, defined within the body and extendinglaterally therein adjacent to the leading edge, and opening to each ofthe first and second lateral sides; a second tubular conduit configuredto receive a cable therethrough, defined within the body and extendinglaterally therein aft of, adjacent to, and parallel to the first tubularconduit, and opening to each of the first and second lateral sides,wherein the first and second tubular conduits are positioned within aforward half of the chord length; and a third tubular conduit configuredto receive a cable therethrough, defined within the body and extendinglaterally therein adjacent to the trailing edge within an aft half ofthe chord length, parallel to the second tubular conduit, and opening toeach of the first and second lateral sides.
 2. The depressor foil ofclaim 1, wherein a form of the body is solid with the exception of thefirst and second tubular conduits.
 3. The depressor foil of claim 1further comprising a fourth tubular conduit configured to receive acable therethrough, defined within the body and extending laterallytherein adjacent to, aft of, parallel to the second tubular conduitwithin a forward half of the chord length, and opening to each of thefirst and second lateral sides.
 4. The depressor foil of claim 1 furthercomprising a tail flap section forming a portion of the trailing edge,extending above and aft of the first surface, and forming an angle withrespect to the chord length.
 5. The depressor foil of claim 4, whereinthe tail flap section comprises between 5% and 25% of the chord length.6. The depressor foil of claim 4, wherein a measure of the angle withrespect to the chord length is between 5 degrees and 30 degrees.
 7. Aseismic array comprising a plurality of lead-in cables; a plurality ofstreamer cables; a plurality of seismic sensors attached to respectivestreamer cables; one or more separation cables attached to aft ends ofone or more groups of lead-in cables, wherein the separation cablesprovide for a maximum separation distance between the aft ends ofadjacent lead-in cables and forward ends of adjacent streamer cables; apair of paravanes each attached at an opposing lateral end of theseismic array; a pair of spur lines each connected between a paravaneand an aft end of an adjacent one of the lead-in cables; and a pair ofribbon-foil depressors pivotably attached to at least a portion of thespur lines, the separation cables, or both, each ribbon-foil depressorfurther comprising a plurality of depressor foils arranged adjacent toeach other in a series; each depressor foil further comprising a bodywith a foil shape having a leading edge and a trailing edge and furtherdefining a cambered first surface extending between the leading edge andthe trailing edge configured to provide negative lift; a relatively flatsecond surface with respect to the first surface extending between theleading edge and the trailing edge; a first lateral side that extendsbetween a first lateral edge of the first surface, a first lateral edgeof the second surface, the leading edge, and the trailing edge; and asecond lateral side that extends between a second lateral edge of thefirst surface, a second lateral edge of the second surface, the leadingedge, and the trailing edge; a first tubular conduit configured toreceive a cable therethrough, defined within the body and extendinglaterally therein adjacent to the leading edge, and opening to each ofthe first and second lateral sides; a second tubular conduit configuredto receive a cable therethrough, defined within the body and extendinglaterally therein aft of and parallel to the first tubular conduit, andopening to each of the first and second lateral sides, where in thefirst and second tubular conduits are positioned within a forward halfof the chord; and a third tubular conduit configured to receive a cabletherethrough, defined within the body and extending laterally thereinadjacent to the trailing edge within an aft half of the chord, parallelto the second tubular conduit, and opening to each of the first andsecond lateral sides.
 8. The seismic array of claim 7, wherein a form ofthe depressor foil body is solid with the exception of the first,second, and third tubular conduits.
 9. The seismic array of claim 7,wherein each depressor foil further comprises a fourth tubular conduitconfigured to receive a cable therethrough, defined within the body andextending laterally therein adjacent to, aft of, and parallel to thesecond tubular conduit within a forward half of the chord, and openingto each of the first and second lateral sides.
 10. The seismic array ofclaim 7, wherein the depressor foil further comprises a tail flapsection forming a portion of the trailing edge, extending above and aftof the first surface, and forming an angle with respect to the chord.11. The seismic array of claim 10, wherein the tail flap sectioncomprises between 5% and 25% of the chord.
 12. The seismic array ofclaim 10, wherein a measure of the angle with respect to the chord isbetween 5 degrees and 30 degrees.
 13. A method of providing downwardlift to a portion of a seismic array comprising providing a ribbon-foildepressor for attachment to a laterally-oriented cable within theseismic array, wherein the ribbon-foil depressor comprises a pluralityof depressor foils arranged adjacent to each other in a series; eachdepressor foil further comprising a body with a foil shape having aleading edge and a trailing edge and further defining a cambered firstsurface extending between the leading edge and the trailing edgeconfigured to provide negative lift; a relatively flat second surfacewith respect to the first surface extending between the leading edge andthe trailing edge; a tail flap section forming a portion of the trailingedge, extending above and aft of the first surface, and forming an anglewith respect to a chord of the foil shape; and a plurality of conduitsformed within the body at a plurality of pivot points located in aforward half of a chord of the foil shape; wherein each of the conduitsis configured to receive the cable therethrough; selecting a pivot pointfrom the plurality of pivot points; pivotably affixing the ribbon-foildepressor to the cable by threading the cable through one of theplurality of conduits located at the selected pivot point in each of thedepressor foils; and towing the seismic array through a body of water.14. The method of claim 13, wherein each depressor foil furthercomprises an aft conduit formed within the body in an aft half of thechord of the foil shape and configured to receive a cable therethrough;and the method further comprises threading a control cable through theaft conduit; and adjusting a length of the control cable to modify acatenary of the ribbon foil depressor along the laterally-orientedcable.
 15. A device for providing downward lift to a cable in a towedmarine seismic array comprising a cable connector configured to attachto the cable; a rigid rod configured to be oriented in parallel with asurface of a body of water through which the array is towed; two or moredepressor foils pivotably mounted on the rigid rod, wherein eachdepressor foil further comprises a body with a foil shape having aleading edge and a trailing edge and further defining a cambered firstsurface extending between the leading edge and the trailing edgeconfigured to provide negative lift; a relatively flat second surfacewith respect to the first surface extending between the leading edge andthe trailing edge; a tail flap section forming a portion of the trailingedge, extending above and aft of the first surface, and forming an anglewith respect to a chord of the foil shape; a first lateral side thatextends between a first lateral edge of the first surface, a firstlateral edge of the second surface, the leading edge, and the trailingedge; and a second lateral side that extends between a second lateraledge of the first surface, a second lateral edge of the second surface,the leading edge, and the trailing edge; a first tubular conduit definedwithin the body and extending laterally therein adjacent to the leadingedge and opening to each of the first and second lateral sides; and asecond tubular conduit defined within the body and extending laterallytherein aft of and parallel to the first tubular conduit and opening toeach of the first and second lateral sides; and a bridle connected at afirst end to opposing ends of the rigid rod and connected at a secondend to the cable connector.
 16. The device of claim 15, wherein thecable connector comprises a clamp configured to fixedly connect with thecable.
 17. The device of claim 15, wherein the cable connector isconfigured to slide along or comprises a sliding mechanism that slidesalong the cable; and the device further comprises a tagline connected tothe cable connector and configured to adjust a position of the cableconnector with respect to the cable and to limit sliding of the cableconnector in an aft direction.